1. Field of the Invention
The present invention relates to an optical fiber coupling structure and to a method for coupling optical fibers used for optical fiber lasers or the like.
Priority is claimed on Japanese Patent Application No. 2004-29406, filed Feb. 5, 2004, the contents of which are incorporated herein by reference.
2. Description of Related Art
In recent years, optical fiber lasers and optical fiber amplifiers have been studied actively. In these devices, an optical fiber using as host glass silica glass doped with rare earth elements, such as erbium (Er), neodymium (Nd), ytterbium (Yb), or holmium (Ho) (hereinafter referred to as “rare earth element doped optical fiber”) or fluoride glass is used as a laser medium.
An optical fiber laser has an advantage in that a single medium can be used for both emission and propagation of a laser, in addition to having high efficiency and enabling size reduction. Because of these advantages, optical fiber lasers have been used in a wide variety of fields, including optical communications, optical sensing technologies, machining of materials, and medical technologies.
FIG. 6 is a schematic diagram of a typical fiber laser. As shown in FIG. 6, a fiber laser 101 includes a rare-earth doped optical fiber 102 and resonator mirror 103a and 103b that are disposed at the two ends of the rare-earth doped optical fiber 102. The length of the rare-earth doped optical fiber 102 is adjusted so that a desired output characteristic is obtained. Having such a structure, the fiber laser 101 functions as an optical resonator.
For example, dielectric multilayer films are used as the resonator mirror 103a and 103b, which reflect output laser light and transmit excitation light. Excitation light is emitted from an excitation light source 104 via the resonator mirrors 103a and 103b. The excitation light source 104 is coupled to a guide fiber 105 that propagates that excitation light, and the light emitted from the guide fiber 105 is incident on the rare-earth doped optical fiber 102.
For industrial applications of optical lasers, it is desired that a laser be guided through an optical fiber and be incident on a desired location. For this reason, a transmission optical fiber 106 is coupled to the output of the rare-earth doped optical fiber 102.
It is known that the laser operation sometimes becomes unstable in a conventional optical fiber laser, such as the optical fiber laser 101, since light is reflected on an end surface of the optical fiber, and some of the output laser light is returned to the resonator or some of the excitation light is incident on the excitation light source 104. In order to reduce the reflection of light, nonreflective dielectric multilayer films are provided on an end surface 105a of the guide fiber and end surfaces 106a and 106b of the transmission fiber.
The end surfaces of such optical fibers are formed as optical connectors so that the optical fibers are coupled with a high precision, and upon coupling optical fibers using the optical connectors, the end surfaces of the optical fibers physically contact each other, which may damage the dielectric multilayer films (see Japanese Unexamined Patent Application, First Publication No. H01-297874, for example).
In view of the above-mentioned problem, the present inventors have proposed some connector structures that can prevent damage to dielectric multilayer films due to coupling or the like. The connector structures are shown in FIGS. 7A and 7B. In the structure shown in FIG. 7A, at ends of the optical fibers 111 (reference numeral 111a denotes an incidence-side optical fiber, and reference numeral 111b denotes an emission-side optical fiber), dielectric multilayer films 112a and 112b are provided, respectively. At one end of the optical fibers 111, ferrules 113a and 113b or the like are provided to construct connectors 114. In order to couple the optical fibers 111, the ferrules 113a and 113b of the connectors 114a and 114b are advanced into a sleeve 115. Upon coupling, when flanges 116a and 116b protruding from the rear end (the end opposing to the end surface) of the ferrules 113a and 113b contact the sleeve 115, the ferrules 113a and 113b prevent from being inserted further. As a result, a small clearance 117 is defined between the dielectric multilayer films 112a and 112b; thus the damage to the dielectric multilayer films 112a and 112b can be prevented.
The structure shown in FIG. 7B is similar to the structure in FIG. 7A in that flanges 116a and 116b of the ferrules 113a and 113b restrict the insertion beyond the limit in order to prevent the damage to the dielectric multilayer films 112. In addition to that, the ends of the ferrules 113a and 113b are abutted against the protrusion provided inside the sleeve 115, thereby preventing the connectors 114a and 114b from being inserted further into the sleeve 115 (see, for example, Japanese Unexamined Patent Application, First Publication No. 2005-3871 by the applicants of the present invention).
In the coupling structures for optical fibers described above, upon coupling the optical fibers 111a and 111b, the spacing between the end surfaces is defined by mechanical engagement among the ferrules 113a and 113b and the sleeve 115. In order to realize an optical coupling with low loss, the internal structures of the ferrules 113a and 113b and the sleeve 115 should be manufactured with a very high precision (for example, on the order of submicrons). More specifically, if a slight misalignment in a horizontal direction (core misalignment) is present in the optical coupling between the optical fibers 111a and 111b, some of the light emitted from the emission-side optical fiber 111a may not be incident on the core of the incidence-side optical fiber 111b. As a result, loss may be incurred and the optical characteristics of the coupling may be adversely affected. For this reason, when coupling the optical fibers 111a and 111b, the outer diameter of ferrules 113a and 113b and the inner diameter of the sleeve 115 should be aligned very precisely. Thus, these parts need to be manufactured with a very high precision, which requires intensive work. In addition, if the clearance 117 is not precisely set, the spot radius of the light emitted from the emission-side optical fiber 111a and then incident on the incidence-side optical fiber 111b may become larger than the core radius of the incidence-side optical fiber 111b. In such a case, a part of the light emitted from the emission-side optical fiber 111a is not incident on the core of the incidence-side optical fiber 111b. As a result, loss may be incurred and the optical characteristics of the coupling may be adversely affected. For this reason, the optical fibers 111a and 111b should be aligned very precisely, and the ferrules 113a and 113b and the sleeve 115 need to be manufactured with a very high precision, which requires intensive work.
Furthermore, thermal curing adhesive is required for attaching the ferrule 113a and the optical fiber 111a, and the ferrule 113b and the optical fiber 111b, which is time consuming.